In this case A, B, and C did not behave exactly the same.

If you think that literally everyone will downvote and/or get downvoted and/or retaliate and/or get retaliated against all in lockstep or that literally no one will and we will all drown in a sea of sameness, we will have to wait and see but I'm very much doubting it.

I'm afraid the incentive to act is still negative.

Somewhat. Certainly there is still minimal if any positive incentive to downvote, and indeed many still may just not bother, or not want to enter the fray. That is a clear potential point of failure. However, all those issues already exist. Compared with now, the overall cost of downvoting is surely reduced dramatically for nearly everyone. How much difference that will make remains to be seen.